1.Field of the Invention
The invention relates to a cast steel piston for internal combustion engines, which consists of a reduced-density steel alloy or of a high-grade steel alloy, or to a steel piston partially cast from ADI or GJV and partially formed from a reduced-density steel alloy or a high-grade steel alloy, and also to a method for producing a one-piece and materially unitary steel piston.
2.Description of the Related Art
On account of the increasing requirements with regard to as high peak pressures as possible in reciprocating internal combustion engines, which amount to up to 250 bar, lightweight aluminum pistons are increasingly reaching their performance limit. Steel pistons are therefore increasingly demanded again for the motor truck sector, but also for the passenger car sector. The stringent requirements in terms of useful life and reliability in this case make it necessary to have, in particular, pistons which are manufactured completely from steel and are to replace the steel and aluminum pistons still often used at the present time.
As compared with aluminum pistons, however, steel pistons have the disadvantage of a higher weight.
The production of pistons manufactured completely from steel is often complicated and costly because of the difficulty of processing steel for filigree components.
Thus, for example, it is customary to carry out the production of the piston by welding two forgings together.
As a result, the use of different materials for the upper part and lower part is also possible.
DE 102 44 513 A1 discloses a method for producing a multipart cooled piston. The piston upper part is manufactured from heat-resistant steel and the piston lower part from forged AFP steel. The subsequent joining or connecting of the annular rib of the piston upper part to the carrying rib of the piston lower part is carried out by means of a welding or soldering method. The preparation of the parts for joining and the joining method itself constitute cost-intensive method steps.
In EP 1612 395 A1, it is proposed to cast the entire piston from steel. It is proposed to use one of the two following steel compositions (in percent by mass) as the casting alloy:    C≦0.8%, Si≦3%, Mn≦3%, S≦0.2%, Ni≦3%, Cr≦6%, Cu≦6%, Nb 0.01-3%, the rest Fe, with unavoidable impurities,    or C≦0.1-0.8%, S≦3%, Si≦3%, Mn≦3%, S≦0.2%, Ni≦10%, Cr≦30%, Cu≦6%, Nb≦0.05-8% and the rest Fe, with unavoidable impurities.
In this case, in particular, the good room temperature yield strength and also high high-temperature tensile strength and breaking strength play a part.
On account of the filigree type of construction of a piston, the flowability of the casting metal and also the casting method must satisfy particularly stringent requirements. The casting method and the flowability of the metal are of critical importance for achieving a suitable and fault-free structure which is indispensible for the high strength requirements of the cast components. Even minimal structural faults and shrinkage cavities in the casting may lead, in the thin walls of the piston, to a catastrophic material failure.